The present invention relates to a punch-riveting die for a punch-riveting tool, comprising a basic body, which has a recess for forming an underside of a punch-rivet joint, the recess having a base and an annular wall.
Further, the present invention relates to a punch-riveting tool comprising a frame, on which a punch-riveting head comprising a punch-riveting punch is mounted so as to be movable in the punching direction, and on which such a punch-riveting die is fixed.
Finally, the present invention relates to a method for producing a punch-rivet joint of at least two workpieces by the use of such a punch-riveting die.
Normally, in the case of such a punch-riveting method, metallic sheet portions are joined to each other, in that a punch rivet is driven into the sheets by means of a punch.
A distinction is generally made in this case between punch-riveting methods that use solid rivets and those that use semi-hollow rivets. In the case of solid-rivet punch-riveting methods, the rivet itself is composed of a very hard material and is driven completely into the workpiece arrangement, in such a way that the punching residues are expelled. The joint is effected in this case in that the workpiece arrangement is pressed together such that material of at least one workpiece is pressed into a radial groove of the solid rivet.
The present case relates substantially to punch-riveting methods that use semi-hollow rivets. In the case of these rivets, the rivet has a head and a hollow shank that adjoins the latter. The rivet in this case is driven into a workpiece arrangement, the hollow shank widening radially and thereby creating a necessary back taper that results in permanent joining of the workpieces. In this case, the end of the shank does not protrude from the workpiece arrangement. Rather, a so-called residual base thickness of the material of the lowermost workpiece layer remains between the end of the rivet shank and the underside of the thus produced punch-rivet joint.
Such a punch rivet and a die for producing a punch-rivet joint are described in the document DE 10 2006 028 537 B3. This document, in one embodiment, proposes a punch-riveting die wherein a base surface is provided with a central cavity, the diameter of which is less than half the diameter of the recess, and which is of a depth that is greater than 0.3 times the outer diameter of the rivet shank. A radius having a value greater than or equal to 0.3 mm is provided at the transition between the cavity and the recess. The cavity is intended to provide for pressure relief in the joining process, with the result that excessive radial stresses in the punch-rivet joint are prevented and the risk of cracking is reduced.
Owing to the relatively great depth of the central cavity, the punch-rivet joints produced by means of such a punch-riveting die are very thick.
Further, it is generally difficult to produce punch-rivet joints that join two and in particular a plurality of workpieces to each other, it being necessary to use relatively long punch rivets.